add infrastructure so that timing source can be a loadable module... next steps are to convert channel.c and chan_iax2.c to use this new API, and to move all the DAHDI-specific timing source code into a new res_timing_dahdi module


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@122062 65c4cc65-6c06-0410-ace0-fbb531ad65f3